Typical state of the art force sensors use strain gauges to measure the amount of strain of an object under an applied force. The strain gauges are usually metallic in laboratory and test applications and integrated silicon (Si) piezoresistive devices in commercial applications. The gauges are attached to an object to which force is applied, usually by fixing each gauge to the object using a bonding agent such as an epoxy. The applied force causes a change in the aspect ratio (i.e., length divided by cross-sectional area) of the metal filament comprising the gauge, changing the resistance of the gauge. In the case of piezoresistive Si devices, the piezoresistance of Si is responsible for the change in resistance in response to the applied strain. Since this change in resistance is small, the gauge is usually incorporated as one of the arms of a Wheatstone bridge such that the output voltage of the bridge, adjusted to be zero under zero applied force, is proportional to the applied force.
Accurate measurement of strain, and thus the force applied to the object, depends upon the quality of the bond between the gauge and the object. However, the reliability of the bond between the gauge and the object can change drastically with temperature, applied force and other variables. In addition, the reliability of the bond is difficult to measure in any case. One prior art solution proposed is U.S. Pat. No. 5,437,197 to Uras et al., the entire content of which is incorporated herein in its entirety by reference. Uras et al. describes a force sensor based upon the principle of inverse magnetostriction, which is defined as a change in the magnetic properties of a substance under applied stress or strain. A magnetic flux is induced in a magnetic circuit by either a permanent magnet or a coil to which an alternating current is supplied. A detection coil sees an induced voltage. When a force is applied, magnetic properties of the circuit are altered, changing the flux and, consequently, the induced voltage. The sensor of Uras et al., however, is expensive and large, making it difficult to use in a variety of applications.